The Sword Coast
If a spell is listed with material components, then then a component pouch is required to cast the spell.
However, a component pouch may be substituted for a casting focus (rods, wands, crystals, staffs, holy symbols, etc.).
If a spell does not require a material component, neither a component pouch or focus are required.
If a spell is listed with a vocal component, the caster must have the ability to speak. Being silenced or otherwise gagged would prevent the spell from being cast.
If a spell is listed with somatic components, the user must be able to move their arm(s). Being bound, entangled, or otherwise paralyzed would prevent the spell from being cast.